Iron Corrosion in CO[sub 2]/Brine at Low H[sub 2]S Concentrations: An Electrochemical and Surface Science Study

Abstract

The effect of H 2 S at the parts per million level on the early stage of iron corrosion in 3 wt % NaCl solutions saturated with CO 2 is investigated using electrochemical and surface science techniques. Small H 2 S concentrations ͑5 ppm͒ have an inhibiting effect on corrosion in the presence of CO 2 . At higher H 2 S concentrations ͑up to 500 ppm͒, the corrosion rate is higher but below the H 2 S-free case. The characterization of the iron surfaces after corrosion uses photoelectron spectroscopy and electron microscopy. For Fe exposed to H 2 S-containing solutions, a sulfur peak ͑S 2p͒ appears at a binding energy of 162.1 eV, attributable to disulfide ͑S 2 2− ͒ adsorption. The Fe 2p 3/2 peak attributed to Fe͑II͒ at the surface shifts by about 1 eV in the presence of 5 ppm H 2 S compared to the H 2 S-free case due to bonding of Fe to S and O. At higher H 2 S concentrations, the formation of a S-rich surface is indicated. Morphological changes are observed on the iron surface after exposure to H 2 S-containing solutions. A thin protective film forms at 5 ppm H 2 S, while at higher H 2 S concentrations a thicker, more porous surface phase forms. A mechanism for corrosion inhibition of iron in different concentrations of H 2 S in CO 2 /brine is proposed.In the search for new sources of oil and gas, operational activities have moved to harsher environments in deeper high pressure/high temperature wells and remote offshore locations. This has created increased challenges to the economy of project development and execution where operational integrity and an accurate prediction of material performance are becoming paramount. The economic move toward multiphase transportation through subsea completions and long in-field flow lines has increased the risk of corrosion. [1][2][3][4][5][6][7][8][9] Corrosion, therefore, remains a major operational obstacle to successful hydrocarbon production, and its optimum control and management is regarded essential for the cost-effective design of facilities and their safe operations. This has wide-ranging implications on the integrity of many materials used in the petroleum industry. 1-9 Oilfield corrosion manifests itself in several forms, among which CO 2 corrosion ͑sweet corrosion͒ and H 2 S corrosion ͑sour corrosion͒ in the produced fluids and oxygen corrosion in water injection systems are by far the most prevalent forms of attack encountered in oil and gas production. The majority of oilfield failures result from CO 2 corrosion of carbon and low alloy steels primarily due to inadequate knowledge and predictive capability and the insufficient resistance of carbon and low alloy steels to this type of attack. 1-12 The understanding, prediction, and control of this corrosion are key challenges to sound facilities design, operation, and subsequent integrity assurance.CO 2 corrosion..-CO 2 corrosion, or "sweet corrosion," of carbon and low alloy steels is not a new problem. It was first recorded in the U.S. oil and gas industry in the 1940s, followed by several studies since then. 1-8 A...

Corrosion of steel in weakly acidic media at low H2S concentrations

“…At a higher concentration of hydrogen sulphide, a porous layer of sulphide or oxysulfide forms due to the conversion of iron oxide into them. Its protective properties are lower than for the adsorption layer formed at the minimum concentration of H 2 S. The data obtained by us agree with the explanations of the inhibitory effect of hydrogen sulphide proposed in [9], although in the M1 medium under investigation there is no carbon dioxide. Obviously, the atmosphere of carbon dioxide promoted the absence of oxygen in the reaction medium.…”

“…There is no effect of the inhibitor on the cathodic process. The protective effect of small concentrations of hydrogen sulphide (from 5 to 500 mg/L) with respect to iron was observed by the authors of [9] in a 3% NaCl solution saturated with CO 2 . The combination of impedance and surface spectroscopic studies allowed them to conclude that at an H 2 S concentration of 5 mg/L, its inhibitory effect is due to sulfur adsorption on the air-formed iron oxide present on the metal surface.…”

Effect of H2S on the CO2 corrosion of carbon steel in acidic solutions

“…More recently, Ma et al claimed that H 2 S provides a strong inhibition under certain special conditions that have lower H 2 S concentration (≤0.04 mmol dm −3 ), pH value of 3-5, and longer immersion time (≥2 h) [6]. Abelev et al also reported that 5 ppm of H 2 S concentration have an inhibiting effect on corrosion in the presence of CO 2 [7]. Even though there is no absolute criterion for the H 2 S concentration that provides inhibition, it has been suggested that the inhibition effect is related to the formation of iron sulfide with different crystal structures, such as amorphous ferrous sulfide, mackinawite, cubic ferrous sulfide, smythite, greigite, pyrrhotite, troilite, and pyrite [8][9][10][11][12].…”

Corrosion mechanism of low‐alloy steel used for flexible pipe in vapor‐saturated H2S/CO2 and H2S/CO2‐saturated brine conditions

“…The corrosion behavior of low‐alloy pipeline steel in H 2 S/CO 2 brine condition is different from the corrosion behavior of low‐alloy pipeline steel exposed to single CO 2 or H 2 S brine condition because of their mutual effect. Many studies reveal that the corrosion behavior of low‐alloy pipeline steel in H 2 S/CO 2 brine condition depends on several factors such as chemical compositions, temperature, total pressure, partial pressure and microstructure …”

“…Many studies reveal that the corrosion behavior of low-alloy pipeline steel in H 2 S/CO 2 brine condition depends on several factors such as chemical compositions, temperature, total pressure, partial pressure and microstructure. [15][16][17][18] Corrosion characteristics of low-alloy pipeline steel in H 2 S/CO 2 brine condition such as electrochemical property, corrosion kinetics, corrosion products and morphology characteristics are different from those of pure CO 2 or H 2 S condition. In conclusion, many scientists studied the corrosion behavior of low-alloy steel in H 2 S/CO 2 brine condition, which was important for the continual study of H 2 S/CO 2 corrosion.…”